The present invention relates to a turning gear for a turbine and, more particularly, to a clutch assembly for use in a turning gear for a steam turbine.
Steam turbines are commonly used to drive electrical generators in power plants. A typical steam turbine is a massive yet intricate piece of machinery that must be started up and shut down in a controlled manner in order to protect each of many steam turbine components from damage due to thermal stresses. Damage due to thermal stresses often results from uneven cooling of components during shutdown or uneven heating of components during startup.
An example of a steam turbine component that is particularly sensitive to thermal stresses during shutdown or startup, is a turbine rotor. For example, the turbine rotor of a steam turbine that is being started up with gland sealing steam cut in will bow or distort if left stationary even for a few minutes. Similarly, a stationary turbine rotor that is cooling down following a shutdown may also bow in a short time. If, however, the turbine rotor of a steam turbine that is being started up or shut down is forced to rotate at a slow speed during the startup or shutdown procedure, the turbine rotor will not bow. Rotation of the turbine rotor during startup and shutdown at the slow speed allows the turbine rotor to endure the thermal stresses without distortion or bowing. In order to provide rotation of the turbine rotor during startup and shutdown, a turning gear is typically installed, which keeps the turbine rotor rotating slowly (e.g., 3–5 revolutions per minute (rpm)) during the startup or shutdown procedure.
The turning gear typically includes an electric motor, reduction gearing and an engagement mechanism. The electric motor typically operates at a high speed of rotation, such as 1100 rpm, and therefore requires reduction gearing to reduce a speed of rotation of the turbine rotor to about 3–5 rpm. The reduction gearing transfers torque from an output shaft of the electric motor to the turbine rotor via a sequence of gears that provide, for example, a 200 to 1 ratio between the speed of rotation the electric motor and the speed of rotation of the turbine rotor. The engagement mechanism provides a mechanism for engaging or disengaging the reduction gearing to the turbine rotor.
It is common for steam turbines to experience a certain amount of steam leakage. In some cases, the amount of steam leakage is enough to increase the speed of rotation of the turbine rotor. In the past, when the speed of rotation of the turbine rotor increased above that which was the result of torque from the turning gear, a torque was transmitted from the turbine rotor to the reduction gearing. The reduction gearing then fed the torque to the electric motor by turning the output shaft of the electric motor, for example, at 200 times the speed of the turbine rotor. The torque fed to the electric motor was often sufficient to overspeed and damage or destroy reduction gearing closest to the electric motor, and therefore also, the turning gear.
To prevent damage or destruction of the turning gear, an overspeed protection was developed which allowed the turning gear to disengage the turbine rotor if the speed of rotation of the turbine rotor reached a predetermined level. While such protection has proved useful in preventing damage or destruction of the turning gear, such protection is not without faults. If an operator fails to notice disengagement of the turning gear, the rotor may eventually coast to a stop if the steam leakage is subsequently reduced, thereby allowing the turbine rotor to become bowed or distorted. If the operator notices disengagement of the turning gear, the turning gear may be re-engaged, however, this process could be repeated extensively thereby decreasing operator attention to other important aspects of operation. Furthermore, an operator may bypass safety controls that prevent re-engagement and force a rotating turbine rotor to damage the turning gear.
Thus, it is desired to develop a turning gear that overcomes the above-mentioned shortcomings.